Bad idea: Tempted by a silicon valley startup because that's what all the cool kids were doing. They were style over substance.
Good idea: leaving my otherwise excellent first real job. I owe them ( slidebook.com ) a lot. I wouldn't be half the developer i am now if they hadn't given me the environment to approach software as a science. That said, leaving was the right call.
I don't regret working for Amazon... for the most part. But that place also messed me up for years. Now i can look back on that place as a learning experience. I hope you get there too.
Would i do it again? No.